Method for producing an optoelectronic device and optoelectronic device

Abstract

A method for producing an optoelectronic device is provided, the method comprising: providing a semiconductor chip (10) that, in operation, emits electromagnetic radiation from a radiation exit surface (15), wherein the radiation exit surface (15) of the semiconductor chip (10) comprises a roughness (R) of at least 0.1 µm and at most 2 µm, forming a smoothened surface (16) on the radiation exit surface (15), and directly attaching a converter element (20) to the smoothened surface (16). Furthermore, an optoelectronic device is provided. The optoelectronic device may be an LED, for example, a micro-LED.

Description

[0001] Description

[0002] METHOD FOR PRODUCING AN OPTOELECTRONIC DEVICE AND

[0003] OPTOELECTRONIC DEVICE

[0004] A method for producing an optoelectronic device and an optoelectronic device are disclosed .

[0005] Embodiments provide a method for producing an optoelectronic device with improved performance . Further embodiments provide an optoelectronic device with improved performance .

[0006] According to at least one embodiment , a method for producing an optoelectronic device is provided .

[0007] According to at least one embodiment , the method comprises providing a semiconductor chip that , in operation, emits electromagnetic radiation from a radiation exit surface .

[0008] The electromagnetic radiation may comprise , for example , visible light , i . e . , radiation with wavelengths chosen from about 380 nm to 780 nm, or ultraviolet light of the wavelength range of 200 nm to 380 nm, or infrared light of the wavelength range of 780 nm to 5000 nm . Here and in the following, the expression " light" may also be used for electromagnetic radiation .

[0009] The semiconductor chip may be a light emitting diode ( LED) chip or a laser diode chip . In particular, the semiconductor chip comprises an epitaxially grown semiconductor layer sequence with an active region being able to generate electromagnetic radiation when being operated . " In operation" means , here and in the following, when current is applied to the device. For example, the active region comprises a pn- junction or a quantum well structure. Exemplary semiconductor materials of the semiconductor chip comprise InGaN, GaN, AlGaN, AIN and InAlGaN, InGaAlP, and GaAs .

[0010] A radiation exit surface is the surface of the semiconductor chip from which the electromagnetic radiation leaves the semiconductor chip or is outcoupled. For example, the semiconductor chip may comprise a bottom surface with which it is arranged on a substrate or on the bottom of a housing, and a top surface being opposite to the bottom surface. In this case, the top surface corresponds to or is at least part of the radiation exit surface, i.e., the surface of the semiconductor chip from which the radiation is outcoupled. Such an optoelectronic device may also be called a topemitting optoelectronic device.

[0011] According to at least one embodiment, the radiation exit surface of the semiconductor chip comprises a roughness of at least 0.1 pm and at most 2 pm, in particular, of at least 1 pm and at most 2 pm, for example, of 1.5 pm. Thus, the radiation exit surface of the semiconductor chip is not a smooth surface, but a rough surface. A rough surface may also be called a structured surface. A rough surface may enhance the light extraction from the semiconductor chip through the radiation exit surface and, thus, the performance of the optoelectronic device.

[0012] The value of the roughness, i.e., between 0.1 pm or more and 2 pm or less, indicates the average difference in height of the roughness. Thus, the semiconductor chip may have a thickness vertically to the radiation exit surface, which varies between a lowest level and a highest level, and the highest level is between 0 . 1 gm and 2 gm thicker than the lowest level . Furthermore , the roughness of the radiation exit surface is part of the semiconductor layer sequence of the semiconductor chip . Thus , the material forming the roughness is the material of the semiconductor chip, in particular of the outer layer of the semiconductor layer sequence of the semiconductor chip .

[0013] According to at least one embodiment , the method further comprises forming a smoothened surface on the radiation exit surface . This step serves to flatten the roughness of the radiation exit surface . Therefore , with this step the radiation exit surface is smoothened . However, this step can be performed without mechanically modi fying the radiation exit surface . Thus , also after the step of forming a smoothened surface , the semiconductor chip comprises the radiation exit surface comprising the roughness of at least 0 . 1 gm and at most 2 gm, while there is an additional smoothened or planari zed surface on the radiation exit surface comprising a reduced or no roughness . Thus , according to at least one embodiment , the smoothened surface covers the radiation exit surface . With this , the radiation exit surface still enhances a good light extraction as the smoothened surface comprises the radiation exit surface with its roughness of at least 0 . 1 gm and at most 2 gm .

[0014] According to at least one embodiment , the method further comprises directly attaching a converter element to the smoothened surface .

[0015] A converter element means here and in the following a completed element having an arbitrary geometry and comprising one surface that is attached to the semiconductor chip . For example , a converter element has the form of a layer or of a platelet which is applied and attached with one of its surfaces to the smoothened surface .

[0016] A converter element is to be understood as an element that is able to absorb electromagnetic radiation of a first wavelength range ( also called primary radiation) , i . e . , the radiation emitted by the semiconductor chip, and emit electromagnetic radiation of a second wavelength range ( also called secondary radiation) . To reali ze the conversion, the converter element comprises or consists of at least one kind of phosphor material or of many kinds of phosphor materials . For the conversion, the electromagnetic radiation emitted by the semiconductor chip is outcoupled from the radiation exit surface and enters from there into the converter element . The at least partially converted electromagnetic radiation is outcoupled from the converter element at its surface facing away from the semiconductor chip .

[0017] "Attaching" means here and in the following a mechanical fixation, which, in particular, is irreversible . A direct attachment comprises , for example , a glueless attachment . Thus , no gluing layer is present between the semiconductor chip and the converter element . In particular, the smoothened surface is in mechanical contact with the converter element after its direct attachment .

[0018] According to at least one embodiment , a method for producing an optoelectronic device is provided, the method comprising : providing a semiconductor chip that , in operation, emits electromagnetic radiation from a radiation exit surface , wherein the radiation exit surface of the semiconductor chip comprises a roughness of at least 0 . 1 pm and at most 2 pm, forming a smoothened surface on the radiation exit surface , and directly attaching a converter element to the smoothened surface .

[0019] Due to the very low thermal conductivity of typical glue materials like silicone that are conventionally used for attaching converters to , for example , light emitting diode ( LED) chips , the converter material is heated when the LED is operated . The high temperature , however, causes a quick degradation of the LED package , leading to shorter li fetimes , especially in the case of high-power LEDs .

[0020] So far, methods are known for directly attaching smooth surfaces to one another without a glue . However, only a surface roughness of about between 1 to 10 nm, in particular 1 to 5 nm, is acceptable , when performing these methods .

[0021] Thus , they are used for attaching smooth surfaces like polished sapphire substrates of LEDs , lasers or quartz oscillators .

[0022] However, in the case of top emitting devices , for example LEDs , a rough surface is desirable or even necessary, in order to have a satis fying light extraction and to improve the light extraction ef ficiency on the radiation exit surface , i . e . , the top surface . Thus , to apply a known method for direct , in particular glueless attachment of a converter element to a semiconductor chip having a rough radiation exit surface , a flattening of the radiation exit surface would be necessary . Flattening such a radiation exit surface of semiconductor chip by removing the rough surface would reduce the light output signi ficantly due to the lack of the surface structure, i.e., the roughness, and is therefore not to considered .

[0023] The inventors have realized a way to apply methods of direct attach of a converter element to a semiconductor chip even if its radiation exit surface comprises a roughness. In particular, they have found out that by forming a smoothened surface methods of direct attach may be applied while the necessary roughness of the radiation exit surface is maintained. Thus, with a method as described here, a direct attachment of a converter element to a semiconductor chip having a rough radiation exit surface can be realized, avoiding conventional glues like silicone having a low thermal conductivity. Therefore, an optoelectronic device produced with a method as described here, can better prevent over-heating of the converter element, and consequently also other materials of the LED, e.g., white reflectors, as no material with low thermal conductivity is between the semiconductor chip and the converter element. Thus, the lifetime of the optoelectronic device produced with this method can be enhanced. At the same time, the roughness of the radiation exit surface of the semiconductor chip is maintained, such that the optoelectronic device produced with this method has an improved light extraction efficiency.

[0024] According to at least one embodiment, the roughness is a surface microstructure comprising bumps and recesses. In particular, the bumps and recesses are present on the radiation exit surface in an alternating manner. The height of a bump with regard to a lowest level of a recess corresponds to the roughness, i.e., the height is chosen from including 0.1 pm to including 2 pm. Such a microstructure can be called wigwam structure and is appropriate for improving the radiation or light extraction ef ficiency from the radiation exit surface .

[0025] According to at least one embodiment , forming a smoothened surface comprises depositing a flattening material at least in the recesses of the surface microstructure to form a flattening layer . In other words , a flattening material is at least partially applied on the radiation exit surface . Therefore , at least the recesses of the microstructure are covered with flattening material . Alternatively, additionally the bumps of the microstructure are covered, so that a continuous layer of flattening material that covers the radiation exit surface is formed . In both cases , the roughness of the radiation exit surface is maintained, but the roughness of the smoothened surface is reduced in order to make the smoothened surface appropriate for the direct attachment step . That is , the flattening layer on the radiation exit surface forms the smoothened surface . Thus , the flattening layer may have the function of a planari zation layer .

[0026] According to at least one embodiment , the flattening layer is formed with a thickness in a range of at least 0 . 1 pm and at most 5 pm . In particular, the flattening layer is formed with a minimum thickness that corresponds to the roughness of the radiation exit surface . Thus , i f the roughness is , for example , 1 . 5 pm, flattening material is applied in an amount so that a flattening layer with a thickness of at least 1 . 5 pm is formed . Furthermore , the maximum thickness of the formed flattening layer is thin enough in order to enable thermal conductivity and heat reduction in the converter element . According to at least one embodiment , forming a smoothened surface additionally comprises polishing the flattening layer . With an additional polishing step, a roughness which may be still present after applying the flattening material may be further reduced . For example , the smoothened surface may comprise , in particular after polishing, a roughness of less than 10 nm, in particular less than 2 nm . Such a low roughness allows the following direct attachment step of the converter element .

[0027] According to at least one embodiment , polishing is performed with a method chosen from mechanical polishing, chemical polishing, wet etching, plasma etching or combinations thereof .

[0028] According to at least one embodiment , the flattening material is chosen from oxides like SiO2, AI2O3, GeO2, and MgO, and fluorides like MgF2, CaF2, YbF3, and YLiF4, and the precursors thereof . A precursor is to be understood as a material , which may be trans formed to the desired flattening material via chemical reactions . An exemplary precursor for SiO2may be TEOS ( tetraethylorthosilicate ) , for example . These flattening materials can be applied as thin layers , in order to form the flattening layer as described here . Furthermore , these materials have a higher thermal conductivity than conventional gluing materials like silicone and enhance , thus , heat reduction in the converter element . Additionally, these materials comprise a favorable refractive index enhancing light extraction .

[0029] According to at least one embodiment , the deposition of flattening material is performed with a method chosen from plasma deposition and chemical vapor deposition . For example , TEOS is chosen as a precursor for SiCy and is applied via chemical vapor deposition.

[0030] According to at least one embodiment, attaching the converter element to the smoothened surface is performed gluelessly. Thus, no gluing layer and thus, no material of low thermal conductivity, is applied between the semiconductor chip and the converter element and heat generated in the converter element can be efficiently transferred to the chip that is usually connected to a heat sink, if necessary. This can result in prevention of over-heating of the LED and as the result, it can prolong the product lifetime of the LEDs.

[0031] According to at least one embodiment, attaching the converter element to the smoothened surface is performed with a method chosen from atomic diffusion bonding (ADB) and surface activated bonding (SAB) . When using SAB, at least one of the surfaces to be attached, i.e., the smoothened surface and a surface of the converter element, are sputter-etched and activated using an Ar ion beam. When using ADB, thin metal films are applied on at least one of the surfaces to be attached, i.e., the smoothened surface and a surface of the converter element, using sputter deposition. In aim of oxidizing the thin metal films to form their oxides, the LED may be, after the assembly, heated in atmospheric environments containing oxygen and oxide gases, as well as in vacuum. Subsequently the surfaces to be attached are bonded under vacuum. Both methods allow a glueless attachment and, thus, the direct attachment of the converter element on the smoothened surface of the semiconductor chip. According to at least one embodiment, only the smoothened surface is surface treated according to the SAB or ADB method before being directly attached to the converter element. Alternatively, according to another embodiment , both, the smoothened surface and a surface of the converter element that is to be attached to the smoothened surface are treated according to the SAB or ADB method before being directly attached to one another .

[0032] According to at least one embodiment , at least one surface of the converter element is polished before being attached to the smoothened surface . Thus , i f necessary, also the surface of the converter element that is to be directly attached to the smoothened surface , can be treated in order to have a roughness of lower than 10 nm, in particular lower than 2 nm . Polishing a surface of the converter element can be performed with a method chosen from mechanical polishing, chemical polishing, wet etching, plasma etching or combinations thereof .

[0033] According to at least one embodiment , polishing the smoothened surface and polishing at least one surface of the converter element is performed at once .

[0034] According to at least one embodiment , an optoelectronic device is provided . The method as described here is suitable and intended to produce an optoelectronic device . Thus , all features and embodiments disclosed with respect to the method for producing an optoelectronic device are also valid for the optoelectronic device and vice versa .

[0035] According to at least one embodiment the optoelectronic device comprises a semiconductor chip that , in operation, emits electromagnetic radiation from a radiation exit surface . The electromagnetic radiation emitted by the semiconductor chip may be or at least comprise visible light , for example , with wavelengths from the range of 380 nm to 700 nm, or ultraviolet light of the wavelength range of 200 nm to 380 nm, or infrared light of the wavelength range of 780 nm to 5000 nm .

[0036] In particular, the semiconductor chip comprises an epitaxially grown semiconductor layer sequence with an active region being able to generate electromagnetic radiation . For example , the active region comprises a pn-j unction or a quantum well structure . Exemplary semiconductor materials of the semiconductor layer sequence are InGaN, GaN, AlGaN, AIN, InAlGaN, InGaAlP, and GaAs .

[0037] The semiconductor chip may be a light emitting diode chip or a laser diode chip .

[0038] A radiation exit surface is the surface of the semiconductor chip from which the electromagnetic radiation is outcoupled from the semiconductor chip . For example , the semiconductor chip may comprise a bottom surface with which it is arranged on a substrate or on the bottom of a housing, and a top surface being opposite to the bottom surface . In this case , the top surface corresponds to or is at least part of the radiation exit surface , i . e . , the surface of the semiconductor chip from which the radiation is outcoupled . Such an optoelectronic device may also be called a topemitting optoelectronic device .

[0039] According to at least one embodiment the radiation exit surface of the semiconductor layer sequence comprises a roughness of at least 0 . 1 pm and at most 2 pm, in particular of at least 1 pm to at most 2 pm, for example of 1 . 5 pm . Thus , the radiation or light extraction from the semiconductor chip through the radiation exit surface can be enhanced by the rough radiation exit surface .

[0040] According to at least one embodiment , the optoelectronic device further comprises a smoothened surface on the radiation exit surface . A smoothened surface has a reduced roughness in comparison to the roughness of the radiation exit surface . In particular, the smoothened surface comprises a roughness of at most 10 nm, for example of at most 2 nm . At the same time , the roughness in a range of at least 0 . 1 pm and at most 2 pm of the radiation exit surface is still present , so that light extraction from the semiconductor chip is enhanced . According to at least one embodiment , the smoothened surface comprises the radiation exit surface having a roughness of at least 0 . 1 pm and at most 2 pm .

[0041] According to at least one embodiment , the optoelectronic device comprises a converter element being directly attached to the smoothened surface .

[0042] A converter element is to be understood as an element that is able to absorb electromagnetic radiation of a first wavelength range ( also called primary radiation) , i . e . , the radiation emitted by the semiconductor chip, and emit electromagnetic radiation of a second wavelength range ( also called secondary radiation) . The converter element can convert the light emitted by the semiconductor chip at least partially or fully . Alternatively, the converter element can convert the light emitted by the semiconductor chip partially while another part is transmitted, such that the device emits a mixed light comprising primary and secondary radiation . The absorbed radiation has, in particular, a wavelength maximum that is different from, for example smaller than, the wavelength maximum of the emitted radiation. Such a process is called wavelength conversion. In particular, scattering or absorption alone is not meant with the term "wavelength conversion" at present. For example, the wavelength maximum of the absorbed radiation is in the UV or blue spectral range, and the wavelength maximum of the emitted radiation is in the yellow, green or red spectral range, i.e., at longer wavelengths than the absorbed radiation. Such a process is called down-conversion.

[0043] To realize the conversion, the converter element comprises or consists of at least one kind of phosphor material or of many kinds of phosphor materials. Thus, the converter element may comprise or consist of a ceramic material or may comprise phosphor particles that are embedded in a matrix material like polymers or glass or other sorts of ceramics, for example. A phosphor is to be understood as the material in the converter element that is responsible for the converting properties, i.e., the wavelength conversion, as explained above. In other words, the phosphor absorbs electromagnetic radiation, which is emitted by the semiconductor chip, for example a light emitting diode (LED) , converts it by a molecular and / or an atomar mechanism, and re-emits it at, for example, longer wavelengths. The phosphor has, thus, photoluminescent properties. Exemplary phosphor materials may be chosen from Y3Al50i2:Ce, LU3AI5O12 : Ce, (Y, Gd) 3 (Al, Ga) 5O12 : Ce, (Lu,Ga) 3 (Al,Ga)50i2:Ce, ( SrxBai-2Si5N8: Eu (0.0<x<1.0) , SrB4O7:Eu, BaMgAl10Oi7: Eu, (Sr, Ba)2SiO4: Eu, ( Y, Gd, Tb, Lu) AG : Ce, (Ca, Sr)2SiO4:Eu, (Ca,Sr)S:Eu, (Ca, Sr ) AlSiN3: Eu and combinations thereof. Being directly attached means that there is no other layer, in particular no gluing layer, between the smoothened surface and the converter element . Thus , the smoothened surface and the converter element have a common boundary surface .

[0044] According to at least one embodiment , an optoelectronic device is provided, the optoelectronic device comprising : a semiconductor chip that , in operation, emits electromagnetic radiation from a radiation exit surface , wherein the radiation exit surface of the semiconductor layer sequence comprises a roughness of at least 0 . 1 pm and at most 2 pm, a smoothened surface on the radiation exit surface , and a converter element being directly attached to the smoothened surface .

[0045] Such an optoelectronic device comprises a high light extraction ef ficiency due to the roughness of the radiation exit surface . Additionally, due to the direct attachment of the converter element to the smoothened surface the optoelectronic device lacks a gluing layer and, thus , a material of low thermal conductivity between the semiconductor chip and the converter element . Therefore , heat can be reduced in the converter element which reduces degradation and, hence , enhances a long li fetime of the optoelectronic device .

[0046] According to at least one embodiment , the roughness of the radiation exit surface is a microstructure comprising bumps and recesses . Such a microstructure may also be called wigwam structure and is suitable for enhancing light extraction from the radiation exit surface . According to at least one embodiment, the smoothened surface comprises a flattening layer. Accordingly, the smoothened surface is at least partially formed of a flattening layer. According to at least on embodiment, the flattening layer is formed on the radiation exit surface, in particular, the flattening layer covers the radiation exit surface. The flattening layer on the radiation exit surface forms, thus, a smoothened surface, while the roughness of the radiation exit surface is maintained. In particular, the flattening layer covers the radiation exit surface at least partially, in particular fully, and is therefor able to planarize the roughness of the radiation exit surface.

[0047] According to at least one embodiment, the flattening layer comprises a flattening material chosen from SiO2, AI2O3, GeO2, MgO, MgF2, CaF2, YbF3, YLiF4. These materials comprise a favorable refractive index, and enhance, thus, light extraction. Furthermore, they comprise a higher thermal conductivity than typical glue materials like silicone, and are, thus, suitable to be applied between the semiconductor chip and the converter element. At the same time, these materials are suitable for forming thin layers. Thus, their thermal conductivity may be maintained.

[0048] According to at least one embodiment, the flattening layer comprises a thickness in the range of at least 0.1 pm and at most 5 pm. Thus, the flattening layer is a thin layer which may enhance heat reduction.

[0049] According to at least one embodiment, the optoelectronic device is free of a gluing layer between the converter element and the semiconductor chip. Thus, no material of low thermal conductivity is present between the semiconductor chip and the converter element , so that heat in the converter element can be reduced when the optoelectronic device is operated .

[0050] According to at least one embodiment the semiconductor chip is applied on a heat sink with its surface facing away from the radiation exit surface . A heat sink may comprise a material chosen from silicon, silicon nitrides , aluminum oxides , aluminum nitrides .

[0051] According to at least one embodiment , the optoelectronic device further comprises a substrate or a housing . In particular, the semiconductor chip is applied on the substrate or on the bottom of the housing with its surface facing away from the radiation exit surface . According to at least one embodiment , the substrate or the bottom of a housing comprises the heat sink .

[0052] According to at least one embodiment the optoelectronic device is a light emitting diode ( LED) , in particular a micro-LED .

[0053] As a rule , a growth substrate is removed from micro-LEDs , so that typical heights of such micro-LEDs are in the range of 1 . 5 pm to 10 pm, for example . In principle , a micro-LED does not necessarily have to have a rectangular radiation exit surface . Generally, for example , an LED could have a radiation exit surface in which, in plan view of the layers of the semiconductor layer sequence , any lateral extent of the radiation exit surface is less than or equal to 100 pm or less than or equal to 70 pm . For example , in the case of rectangular micro-LEDs , an edge length - especially in plan view of the layers of the semiconductor layer sequence - is smaller than or equal to 70 qm or smaller than or equal to 50 qm . Mostly, such micro-LEDs are provided on wafers with - for the micro-LED non-destructively - detachable holding structures .

[0054] Advantageous embodiments and developments of the method and the optoelectronic device will become apparent from the exemplary embodiments described below in conj unction with the figures .

[0055] Fig . 1 shows a schematic cross section of an optoelectronic device before attaching a converter element .

[0056] Fig . 2 shows a schematic cross section of an optoelectronic device according to a reference example .

[0057] Fig . 3 shows a schematic cross section of an optoelectronic device according to an exemplary embodiment .

[0058] In the exemplary embodiments and figures , similar or similarly acting constituent parts are provided with the same reference symbols . The elements illustrated in the figures and their si ze relationships among one another should not be regarded as being true to scale . Rather, individual elements may be represented with an exaggerated si ze for the sake of better representability and / or for the sake of better understanding .

[0059] Figure 1 shows a schematic cross section of an optoelectronic device before attaching a converter element 20 . The optoelectronic device comprises a semiconductor chip 10 . In this exemplary embodiment , the semiconductor chip 10 comprises GaN as semiconductor material and a quantum well 11. The top surface of the semiconductor chip 10 has a roughness R in the form of a microstructure comprising bumps 12 and recesses 13, the microstructure also being called a wigwam structure. The roughness R is in a range of 0.1 pm or more and 2 pm or less, i.e., the height of the bumps 12 in comparison to the lowest level of the recesses 13 is at least 0.1 pm and at most 2 pm, for example 1.5 pm. The roughness R enhances radiation or light extraction from the semiconductor chip 10, thus, the top surface of the semiconductor chip 10 corresponds to the radiation exit surface 15 of the optoelectronic device. The semiconductor chip 10 is with its side facing away from the radiation exit surface 15 applied to a heat sink 50.

[0060] In order to at least partially convert the electromagnetic radiation emitted by the semiconductor chip 10 into an electromagnetic radiation of a different wavelength, a converter element 20 has to be applied on the semiconductor chip 10. Depending on the desired color of the total emission of the optoelectronic device, a suitable phosphor is chosen for the converter element 20. The converter element may comprise the phosphor and a matrix material, such as glass or a polymer, or it may consist of a phosphor. Exemplary phosphor materials can be chosen from Y3Al50i2:Ce, LU3AI5O12 : Ce, (Y,Gd) 3 (Al,Ga)50i2:Ce, (Lu, Ga) 3 (Al, Ga) 5O12 : Ce, (SrxBai-2Si5N8:Eu (0.0<x<1.0) , SrB4O7:Eu, BaMgAl10Oi7: Eu, (Sr, Ba)2SiO4:Eu, ( Y, Gd, Tb, Lu) AG : Ce, (Ga, Sr )2SiO4: Eu, (Ca,Sr)S:Eu, (Ga, Sr ) AlSiN3: Eu and combinations thereof.

[0061] With a radiation exit surface 15 comprising the roughness R as shown in Figure 1, conventionally a converter element 20 could only be applied on the radiation exit surface 15 via a gluing layer 40. Methods for direct attach of a converter element 20, i.e., without a gluing layer, need smooth surfaces with a roughness R smaller than 10 nm, in particular smaller than 2 nm.

[0062] Figure 2 shows as a reference example, such a conventional device. As can be seen, a gluing layer 40, in this example a silicone layer, is applied on the radiation exit surface 15, and a converter element 20 is attached to the semiconductor chip 10 via the gluing layer 40. However, such a device shows a quick degradation as the gluing layer 40 has a low thermal conductivity leading to an excess-heating of the converter element 20, and eventually to an excess heating of all the materials of the LED. Therefore, this may lead to the material degradation caused by the excess heating. Such a conventional optoelectronic device has a shortened lifetime.

[0063] Figure 3 shows an exemplary embodiment avoiding a gluing layer 40 and maintaining the roughness R in the range of 0.1 pm or more and 2 pm or less of the radiation exit surface 15. This optoelectronic device is produced according to a method as described here. It has, due to the radiation exit surface 15 with its roughness R of at least 0.1 pm and at most 2 pm a high light extraction efficiency. At the same time, due to the lack of a conventional gluing layer, heat can be reduced in the converter element 20, so that the lifetime of such an optoelectronic component is enhanced.

[0064] According to Figure 3, an optoelectronic device has on the radiation exit surface 15, i.e., on the roughness R or the microstructure comprising bumps 12 and recesses 13, respectively, a thin flattening layer 30. The flattening layer 30 comprises a minimum thickness that corresponds to the roughness R, that is, to the height of the bumps 12. Generally, a flattening layer 30 may have thickness of 0 . 1 gm to 5 gm, depending on the value of the roughness R, which in this example is 1 . 5 gm, but can be chosen from the range of 0 . 1 gm to 2 gm inclusive .

[0065] In this example , the flattening layer 30 is made of SiCg . When producing the flattening layer 30 , a flattening material is applied on the radiation exit surface 15 via plasma deposition or chemical vapor deposition . The flattening layer 30 of Sieg is , for example , produced by applying the precursor TEOS via chemical vapor deposition . However, the application of other flattening materials or their precursors is possible as well . For example , AI2O3, GeCg , MgO, MgF2, CaF2, YbF3, and YLiF4are examples of suitable flattening materials . A flattening material is suitable i f it is applicable in the form of a thin layer . Additionally, it needs to have a refractive index that maintains or enhances light extraction . Furthermore , it should have a high thermal conductivity, in particular a higher thermal conductivity than gluing materials like silicone .

[0066] When the flattening material is applied on the radiation exit surface 15 and a flattening layer 30 is formed thereon, and, thus , a smoothened surface 16 is formed . The smoothened surface 16 surrounds the roughness R which is still present on the radiation exit surface 15 while having a smooth surface itsel f . Any possibly remaining roughness of the smoothened surface 16 may be decreased by additionally polishing the smoothened surface 16 , for example by chemical or mechanical polishing, by wet etching or by a combination of these methods . The roughness of the smoothened surface is at most 10 nm, in particular at most 2 nm . Now, the smoothened surface 16 is ready for a direct attachment , in particular a glueless attachment of the converter element 20 on the semiconductor chip 10 . Before being attached, the converter element 20 may be polished i f necessary, in order to comprise a surface having a roughness of less than 10 nm, in particular less than 2 nm .

[0067] For the direct attachment a method chosen from SAB and ADB is applied . In case of SAB, the smoothened surface 16 and optionally also the surface of the converter element 20 that is to be attached to the semiconductor chip 10 , are treated with an Ar ion beam and then bonded together, for example in a vacuum chamber . In case of ADB, thin metal films are sputter deposited on the smoothened surface 16 and optionally also on the surface of the converter element 20 that is to be attached to the semiconductor chip 10 . Afterwards , the smoothened surface 16 is bonded to the converter element 20 , for example in a vacuum chamber . In aim of oxidi zing the thin metal films to form their oxides , the LED may be , after the assembly, heated in atmospheric environments containing oxygen and oxide gases , as well as in vacuum .

[0068] With both methods optoelectronic devices are obtained, wherein the converter element 20 is directly attached to the smoothened surface 16 . A gluing layer 40 of , for example , silicone , can be omitted . Thus , in the converter element 20 any heat can be reduced due to the higher thermal conductivity of the flattening layer 30 with respect to a gluing layer 40 . Additionally, the optoelectronic device has a high light extraction ef ficiency due to the roughness R of at least 0 . 1 pm and at most 2 pm of the radiation exit surface 15 . This patent application claims the priority of German patent application 102024101321 . 4 , the disclosure content of which is hereby incorporated by reference .

[0069] The features and exemplary embodiments described in connection with the figures can be combined with each other according to further exemplary embodiments , even i f not all combinations are explicitly described . Furthermore , the exemplary embodiments described in connection with the figures may have alternative or additional features as described in the general part .

[0070] The invention is not restricted to the exemplary embodiments by the description on the basis of said exemplary embodiments . Rather, the invention encompasses any new feature and also any combination of features , which in particular comprises any combination of features in the patent claims and any combination of features in the exemplary embodiments , even i f this feature or this combination itsel f is not explicitly speci fied in the patent claims or exemplary embodiments .

[0071] References

[0072] 10 semiconductor chip

[0073] 11 quantum well 12 bump

[0074] 13 recess

[0075] 15 radiation exit surface

[0076] 16 smoothened surface

[0077] 20 converter element 30 flattening layer

[0078] 40 gluing layer

[0079] 50 heat sink

[0080] R roughness

Claims

Claims1. A method for producing an optoelectronic device, the method comprising: providing a semiconductor chip (10) that, in operation, emits electromagnetic radiation from a radiation exit surface (15) , wherein the radiation exit surface (15) of the semiconductor chip (10) comprises a roughness (R) of at least 0.1 pm and at most 2 pm, forming a smoothened surface (16) on the radiation exit surface (15) , and directly attaching a converter element (20) to the smoothened surface (16) , wherein the roughness (R) is a surface microstructure comprising bumps (12) and recesses (13) , wherein forming a smoothened surface (16) comprises depositing a flattening material at least in the recesses (13) of the surface microstructure to form a flattening layer (30) , wherein the flattening material is chosen from SiCy, AI2O3, GeCp, MgO, MgF2, CaF2, YbF3, YLiF4, and the precursors thereof .

2. The method according to the preceding claim, wherein the flattening layer (30) is formed with a thickness in a range of at least 0.1 pm and at most 5 pm.

3. The method according to one of the preceding claims, wherein forming a smoothened surface (16) additionally comprises polishing the flattening layer (30) .

4. The method according to the preceding claim, wherein polishing is performed with a method chosen from mechanicalpolishing, chemical polishing, wet etching, plasma etching or combinations thereof.

5. The method according to one of the preceding claims, wherein the deposition of flattening material is performed with a method chosen from plasma deposition and chemical vapor deposition.

6. The method according to one of the preceding claims, wherein attaching the converter element (20) to the smoothened surface (16) is performed gluelessly.

7. The method according to one of the preceding claims, wherein attaching the converter element (20) to the smoothened surface (16) is performed with a method chosen from atomic diffusion bonding and surface activated bonding.

8. The method according to one of the preceding claims, wherein at least one surface of the converter element (20) is polished before being attached to the smoothened surface(16) .

9. An optoelectronic device, comprising: a semiconductor chip (10) that, in operation, emits electromagnetic radiation from a radiation exit surface, wherein the radiation exit surface (15) of the semiconductor layer chip (10) comprises a roughness (R) of at least 0.1 pm and at most 2 pm, a smoothened surface (16) on the radiation exit surface (15) , and a converter element (20) being directly attached to the smoothened surface (16) , wherein the smoothened surface (16) comprises a flattening layer (30) , wherein the flatteninglayer (30) comprises a flattening material chosen from SiO2, A12O3, GeO2, MgO, MgF2, CaF2, YbF3, YLiF4.

10. The optoelectronic device according to the preceding claim, wherein the flattening layer (30) comprises a thickness in the range of at least 0.1 pm and at most 5 pm.

11. The optoelectronic device according to one of claims 9 to10, the device being free of a gluing layer (40) between the converter element (20) and the semiconductor chip (10) .

12. The optoelectronic device according to one of claims 9 to11, being a light emitting diode, in particular a micro-LED.

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