CREATION OF A RADIATION EXIT WINDOW FOR A PHOTOEMITTER COMPONENT

Abstract

A process allows the creation of an output window (W) for radiation (R) produced within a silicon carbide substrate (1) by a photoemitting component. This is achieved by applying a heat treatment to the substrate to selectively increase its surface roughness within the output window. This process is compatible with the mass production of the photoemitting component and provides good reproducibility of the output characteristics of the radiation produced by each component. (Abstract figure: Figure 2)

Description

Title of the invention: CREATION OF A RADIATION EXIT WINDOW FOR A PHO COMPONENT TOEMITTER Technical field

[0001] The present description relates to the creation of a radiation exit window for a silicon carbide-based light-emitting component. It also relates to a light-emitting component which is provided with a radiation exit window which can be created in this way. Prior art

[0002] The use of crystalline silicon carbide (SiC) to produce radiation-emitting components has many advantages, which arise in particular from the following properties of the material: its chemical inertness, its thermal stability, its thermal conductivity and its wide band gap, particularly in comparison with silicon (Si), which is the most widely used semiconductor material in microelectronics today. Silicon carbide makes it possible, in particular, to produce radiation in a controlled manner in the spectral range of visible light, and also around 1.5 pm (micrometers), for example for telecommunications applications.

[0003] The efficiency of extracting the radiation produced inside the silicon carbide component, possibly intentionally doped, to the outside of this component so that the radiation can be used for the desired application, is then a major issue. Indeed, more efficient extraction of the radiation makes it possible to reduce the energy consumption of the component for an equal quantity of radiation that is usable. Furthermore, it is essential to be able to mass-produce components whose emission efficiency is controlled and reproducible. Technical problem

[0004] An aim of the present invention is therefore to improve the output efficiency of the radiation which is produced within a silicon carbide-based component, towards the outside of this component. In particular, this improvement is sought so as to be able to implement it in a reproducible and controlled manner for large series of manufactured components. Summary of the invention

[0005] To achieve this or another object, a first aspect of the invention provides a method for creating an exit window for radiation which is produced inside a silicon carbide substrate by a light-emitting component, this method comprising: / 1 / forming a carbon layer on a surface of the substrate, selectively outside a limited portion of this surface which is intended to constitute, during use of the component, the exit window towards the outside of the substrate for the radiation produced inside the substrate by the photoemitting component; and / 2 / applying a heat treatment to the substrate, this heat treatment producing an increase in roughness of the surface of the substrate selectively at locations where the surface is not covered by the carbon layer; then / 3 / remove the carbon layer outside the portion of the substrate surface which is intended to constitute the radiation exit window.

[0006] Indeed, the roughness of the surface of the substrate which is increased in the radiation exit window constitutes, inside this window, a surface layer which has an average refractive index value lower than that which is effective inside the substrate. This layer with reduced refractive index value produces an at least partial antireflection effect for the radiation which comes from the inside of the substrate, in particular the radiation which is produced by the component and whose direction of emission crosses the surface of the substrate in the exit window. Thanks to this antireflection effect, the exit of the radiation towards the outside of the component is favored, so that the component has an improved radiation exit efficiency. In this way, energy consumption by the component is reduced, for an equal quantity of radiation which is usable.Such an antireflection effect is effective especially when the surface of the silicon carbide substrate is exposed to air during use of the light-emitting component. Another explanation for the effectiveness of the roughness of the substrate surface in promoting the exit of the radiation is that the variations in incidence of the radiation with respect to the actual surface of the substrate, due to the roughness, reduce the total reflections of the radiation which are internal to the substrate.

[0007] Furthermore, due to the fact that the radiation output window is formed by the heat treatment of step / 2 / by locally increasing the surface roughness of the substrate as an effect of this heat treatment, the method of the invention is compatible with mass production of the photoemitting component, and it provides good reproducibility of the radiation output characteristics which is produced by each component.

[0008] Advantageously, the heat treatment of step / 2 / can be carried out at atmospheric pressure, under argon (Ar), with a temperature which is between 1200°C (degrees Celsius) and 1900°C, preferably between 1500°C and 1800°C, for a duration of between 1 minute and 1 hour.

[0009] In particular embodiments of the invention, step / 1 / may understand the following sub-steps: / 1-1 / deposit a continuous layer of resin on the surface of the substrate; / 1-2 / applying a pyrolysis treatment to the resin layer, this pyrolysis treatment being adapted to transform the resin layer into the carbon layer; / 1-3 / forming a mask on the carbon layer, selectively outside the portion of the surface of the substrate which is intended to constitute the radiation exit window; / 1-4 / etching the carbon layer so that it is selectively removed at locations not covered by the mask; and / 1-5 / optionally, removing the mask.

[0010] The pyrolysis treatment of sub-step / 1-2 / may comprise heating the resin layer to a temperature which is between 700°C and 800°C for a period of between 10 minutes and 1 hour, under a flow of nitrogen (N2) or argon (Ar).

[0011] The carbon layer can be etched in substep / 1-4 / using a reactive etching process with oxygen ions (O).

[0012] Possibly, the mask-forming sub-step / 1-3 / may comprise depositing a layer of a metal or metal alloy on the carbon layer, then removing this layer of the metal or metal alloy selectively in the portion of the substrate surface which is intended to constitute the radiation exit window. In this case, the mask-removing sub-step / 1-5 / may be carried out using a suitable removal method, such as a wet etching method or a plasma etching method.

[0013] In other particular embodiments of the invention, alternative to the previous ones, step / 1 / may comprise the following sub-steps: / 1-1 / by lithography, depositing a layer of a lithographic resin on the surface of the substrate selectively outside the portion of this surface of the substrate which is intended to constitute the radiation exit window; and / 1-2 / apply a pyrolysis treatment to the resin layer, this pyrolysis treatment being suitable for transforming the resin layer into the carbon layer. For these other embodiments of the invention, the carbon layer is directly formed selectively outside the radiation exit window. The lithographic resin used may be photosensitive, i.e. of the UV lithography type, or of the electron irradiation lithography type. The pyrolysis treatment of sub-step / 1-2 / may also comprise heating the resin layer to a temperature which is between 700°C and 800°C for a period of between 10 minutes and 1 hour, under a flow of nitrogen or argon.

[0014] For the method of the invention, the light-emitting component may comprise a light-emitting diode which is located in the substrate, under its surface, so that when using the component, the radiation is emitted by the light-emitting diode and passes through the output window.

[0015] A second aspect of the invention provides a photoemitting component, comprising a silicon carbide substrate, this substrate having a surface which has a first roughness value, in particular arithmetic roughness, inside a portion of the surface which constitutes, during use of the component, an exit window towards the outside of the substrate for radiation produced inside the substrate by the photoemitting component. The surface of the substrate simultaneously has a second roughness value, also arithmetic roughness where appropriate, outside the surface portion which constitutes the radiation exit window, the first roughness value being greater than the second roughness value.

[0016] Such a light-emitting component may be manufactured using a method which is in accordance with the first aspect of the invention.

[0017] The first roughness value may be between 10 nm (nanometer) and 140 nm, in terms of arithmetic roughness, and the second roughness value may be between 0.1 nm and 5 nm, also in terms of arithmetic roughness.

[0018] Generally for the invention, the silicon carbide substrate may be monocrystalline, in particular according to one of the polytypes 3C, 4H or 6H.

[0019] Finally, the light-emitting component of the second aspect of the invention may likewise comprise a light-emitting diode which is located in the substrate, under its surface, so that when the component is in use, the radiation is emitted by the light-emitting diode and passes through the output window. Brief description of the figures

[0020] The characteristics and advantages of the present invention will appear more clearly in the detailed description below of non-limiting exemplary embodiments, with reference to the appended figures among which:

[0021] [Fig.1a] is a cross-sectional view of a substrate used to manufacture a light-emitting component by applying the invention;

[0022] [Fig.lb] corresponds to [Fig.la] for a later step of the method;

[0023] [Fig. le] corresponds to [Fig. la] for a further stage of the process;

[0024] [Fig.ld] corresponds to [Fig.la] for a further step of the process;

[0025] [Fig. le] corresponds to [Fig. la] for a further step of the process; and

[0026] [Fig.2] is a cross-sectional view of a light-emitting diode component as obtained by applying the invention. Detailed description of the invention

[0027] For the sake of clarity, the dimensions of the elements shown in these figures do not correspond to actual dimensions or to dimensional ratios. real. Furthermore, identical references indicated in different figures designate identical elements or which have identical functions.

[0028] According to [Fig.1a], a substrate 1 of monocrystalline silicon carbide (SiC), for example of polytype 4H or 6H, has a surface S. This surface S may have an initial value of arithmetic roughness, denoted Ra0, which is substantially equal to 1 nm, for example. The surface S is covered with a layer of resin, for example a photolithographic resin such as is commercially available. The resin may be applied to the surface S by a spin-coating process, as known to those skilled in the art. The resin layer which is thus obtained is hardened, then pyrolyzed. The pyrolysis treatment used may be heating at approximately 750°C for a period of a few tens of minutes in a non-oxidizing atmosphere, for example under a flow of nitrogen or argon. The resin layer is thus transformed into a carbon layer, which is designated by the reference 2 in the figure.

[0029] A mask 3 is then deposited on the carbon layer 2, for example using an evaporation process. This mask 3 may be composed of a metal such as aluminum (Al), nickel (Ni), chromium (Cr), or another metal alloy. For this, a continuous layer of the metal or metal alloy may first be deposited on the carbon layer 2, then an opening O3 is formed in the layer of metal or metal alloy. The opening O3 determines a radiation exit window W, provided by the invention for a light-emitting component that will be manufactured from the substrate 1. The opening O3 may be produced using a known selective removal process, adapted to the material of the mask 3, such as direct wet etching lithography. Alternatively, the mask 3 may be formed using a lift-off process. The configuration that is shown in [Fig. 1a] is then obtained.

[0030] The surface of the substrate 1 which thus carries the carbon layer 2 and the mask 3 is treated to remove the carbon layer 2 selectively through the opening O3. For this, a material removal method is selected, which is capable of removing the carbon material without significantly altering the metal or metal alloy of the mask 3. For example, the substrate 1 can be treated by a reactive ion etching process, commonly referred to as RIE for "reactive ion etching" in English, for which the etching ions come from an oxygen plasma. The carbon layer 2 is thus removed by oxidation in the opening O3 of the mask 3, while it remains at the locations where it is covered by the mask 3, as shown by [Fig. 1b]. The mask 3 is then removed completely at this stage of the process. Such removal of the mask 3 can be carried out using wet etching, for example using an acid solution. The configuration of [Fig.[the] is obtained in this way. .

[0031] Alternatively, it is possible to directly produce the carbon layer 2 by outside the window W, using a lithographic resin deposition process. This can be a UV or electron lithography process. Thus, the resin layer is formed by lithography on the surface S of the substrate 1, selectively outside the window W. Then it is transformed into the carbon layer 2 using the same pyrolysis treatment as before, leading again to the configuration of [Fig. 1c],

[0032] A micro- or nanostructuring heat treatment is then applied to the substrate 1 for which the monocrystalline silicon carbide material is selectively covered with carbonic material outside the window W. This heat treatment is selected to cause a desired increase in the roughness of the surface S in the window W, while the roughness of the surface S is not significantly increased, or is increased to a degree which is significantly lower, at the locations where this surface S is covered by the carbonic material. For example, this micro- or nanostructuring heat treatment of the surface S by variation of roughness may consist of heating the substrate 1 to a temperature which is substantially equal to 1700°C for half an hour, under argon. The configuration obtained at the end of this heat treatment is shown in [Fig.ld].The surface S, made of the silicon carbide material, then has a roughness which is increased in the window W compared to outside this window W. For the parameters of the roughness structuring heat treatment which have been cited previously, the surface S has an arithmetic roughness value RaW in the window W, which is substantially equal to 60 nm. Outside the window W, the surface S covered with carbon material has an arithmetic roughness value Ra[ which has remained close to its initial value Ra0. This value Ra[ has been determined to be substantially equal to 1 nm. The values ​​RaW and Ra[ have been called respectively the first and second arithmetic roughness values ​​in the general part of the present description.All roughness values ​​cited in this description were measured using an atomic force microscope, or AFM, or using a mechanical profilometer, for example one of the brands DEKTAK® or TENCOR®.

[0033] The carbon layer 2 is then removed where it remains, i.e. outside the window W. For example, the substrate 1 without the mask 3 can be brought into contact with an oxygen plasma (O2) which is capable of eliminating the carbon material by oxidation, without altering the silicon carbide material. The configuration shown in [Fig. 1c] is then obtained.

[0034] [Fig.2] shows a light-emitting diode which has been fabricated in the silicon carbide substrate 1, with its surface S having been provided with a roughness variation structuring as just described. The method of forming the light-emitting diode troluminescent within the substrate 1, in particular by ion implantations to constitute doped zones, is not the subject of the invention, so that it is not necessary to describe it here and reference may be made to documents already published on this subject. The window W is preferably in line with the light-emitting diode, in a direction which is orthogonal to an average level of the surface S. In a known manner, the light-emitting diode may comprise the following elements: a first zone ZI which is internal to the substrate 1 and doped with N type, for example with nitrogen ions, a second zone Z2, which is also internal to the substrate 1, doped P-type for example with aluminum ions (Al), and contiguous to the zone ZI while being intermediate between the latter and the surface S, a third electrical connection zone, denoted Z3, which is P-type doped and which establishes an electrical connection between zone Z2 and surface S of substrate 1, a rear electrode, referenced 4 and possibly made of nickel (Ni), which is in electrical contact with zone Z1, possibly through an intermediate zone not shown and doped with N type, and an upper electrode 5, which is possibly made of an alloy of aluminum (Al) and titanium (Ti), and which is carried by the surface S so as to be in electrical contact with the zone Z3.

[0035] In [Fig.2], the upper electrode 5 occupies a central part of the window W while being smaller than the latter, so that the surface S of the substrate 1 remains uncovered in a sufficient part of the window W. Alternatively, the upper electrode 5 can occupy a peripheral part of the window W while leaving the surface S of the substrate 1 uncovered in a sufficient central part of the window W. Alternatively again, transparent or semi-transparent metal layers, a few nanometers thick, can be used to constitute the upper electrode 5, in which case such layers can cover the window W.

[0036] Such a light-emitting diode produces radiation from the junction J between the zones Z1 and Z2, when it is supplied with electric current in the forward direction by the electrodes 4 and 5. When the silicon carbide substrate 1 is of polytype 4H, this radiation belongs to the visible light range and is particularly blue in color. When the substrate 1 is of polytype 6H, the radiation produced is more blue / green in color. White light illumination can also be obtained.

[0037] The increased roughness which has been created according to the invention in the window W constitutes a layer with an anti-reflective effect. This anti-reflective effect may only be partial, but it promotes an exit through the surface S, of the radiation which is produced by the light-emitting diode within the substrate 1. As an illustration of the effectiveness of the invention, [Fig.2] shows a first light ray R which comes from the junction J propagating towards the window W. This ray R then crosses the surface S to exit the substrate 1. It can therefore be used outside the substrate 1 in accordance with the desired application for the light-emitting diode. The ray R', which also comes from the junction J but is directed outside the window W, is reflected internally to the substrate 1.

[0038] It is understood that the invention can be reproduced by modifying secondary aspects of the embodiments which have been described in detail above, while retaining at least some of the advantages mentioned. In particular, all the numerical values ​​which have been cited have been cited only by way of example, and can be changed depending on the application considered.

Claims

Claims

1. A method for creating an exit window (W) for radiation (R) which is produced inside a silicon carbide substrate (1) by a light-emitting component, the method comprising: / 1 / forming a carbon layer (2) on a surface (S) of the substrate (1), selectively outside a limited portion of said surface which is intended to constitute, during use of the component, the exit window (W) towards the outside of the substrate for the radiation (R) produced inside said substrate by the light-emitting component; / 2 / applying a heat treatment to the substrate (1), said heat treatment producing an increase in a roughness of the surface (S) of the substrate selectively at locations where said surface is not covered by the carbon layer (2); then / 3 / removing the carbon layer (2) outside the portion of the surface (S) of the substrate (1) which is intended to constitute the exit window (W) of the radiation (R).

2. Method according to claim 1, according to which the heat treatment of step / 2 / is carried out at atmospheric pressure, under argon, with a temperature between 1200°C and 1900°C, preferably between 1500°C and 1800°C, for a duration between 1 minute and 1 hour.

3. Method according to claim 1 or 2, according to which step / 1 / comprises the following sub-steps: / 1-1 / depositing a continuous layer of a resin on the surface (S) of the substrate (1); / 1-2 / applying a pyrolysis treatment to the resin layer, said pyrolysis treatment being adapted to transform the resin layer into the carbon layer (2); / 1-3 / forming a mask (3) on the carbon layer (2), selectively outside the portion of the surface (S) of the substrate (1) which is intended to constitute the exit window (W) of the radiation (R); / 1-4 / etching the carbon layer (2) so that said carbon layer is selectively removed at locations not covered by the mask (3); and / 1-5 / optionally, removing the mask (3).

4. A method according to claim 3, wherein the carbon layer (2) is etched in substep / 1-4 / using an etching method reactive with oxygen ions.

5. Method according to claim 3 or 4, according to which: sub-step / 1-3 / comprises depositing a layer of a metal or a metal alloy on the carbon layer (2), then removing said layer of the metal or the metal alloy selectively in the portion of the surface (S) of the substrate (1) which is intended to constitute the exit window (W) of the radiation (R).

6. Method according to claim 1 or 2, according to which step / 1 / comprises the following sub-steps: / 1-1 / by lithography, depositing a layer of a lithographic resin on the surface (S) of the substrate (1) selectively outside the portion of said surface of the substrate which is intended to constitute the exit window (W) of the radiation (R); and / 1-2 / applying a pyrolysis treatment to the resin layer, said pyrolysis treatment being adapted to transform the resin layer into the carbon layer (2).

7. Method according to any one of claims 3 to 6, according to which the pyrolysis treatment of sub-step / 1-2 / comprises heating the resin layer to a temperature between 700°C and 800°C for a period between 10 minutes and 1 hour, under a flow of nitrogen or argon.

8. A method according to any preceding claim, wherein the light-emitting component comprises a light-emitting diode which is located in the substrate (1), below the surface (S) of said substrate, so that when said component is used, radiation (R) is emitted by the light-emitting diode and passes through the output window (W).

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