DE102011122778B3 - Method for producing a luminescence conversion LED - Google Patents

Abstract

The present invention relates to a method for producing a luminescence conversion LED using a built-in thin film semiconductor converter based on group III nitrides. After growth of the thin film converter, the substrate is removed by laser lift-off. Due to the lacking mechanical stability of the semiconductor layer, which is only a few micrometers thick, the thin-film semiconductor converter has to be applied to the LED chips in a special process. This is done in the proposed method either by means of a shadow mask or a temporary carrier. The proposed luminescence conversion LED has a high conversion efficiency, low thermal quenching and a well tunable emission wavelength and achieves a high light output.

Description

Technical application

The present invention relates to a method for producing a luminescence conversion light-emitting diode, in which a thin layer of a converter material, in particular a semiconductor converter material, is applied to the provided light-emitting diode chips.

In the case of luminescence conversion light-emitting diodes (LUCO LEDs), part or all of the light emitted by one LED is converted by a luminescence converter into light of a different wavelength. The most widespread application is the realization of white LUCO LEDs using a blue pump LED and a yellow luminescence converter, the white light being obtained by additive color mixing from the unconverted blue light component of the pump LED and the light component converted into yellow light.

State of the art

Modern high-performance LEDs based on the group III nitrides are processed as thin-film LEDs, as described, for example, in A. Laubsch et al., "High-Power and High-Efficiency InGaN-Based Light Emitters", IEEE Trans. Electron Devices, 57 (2010), pages 79 to 87 is described. The (AlGaIn) N-based LED structure is epitaxially grown on a sapphire substrate and then transferred by wafer bonding and laser lift-off on a carrier substrate. By using a thin LED semiconductor layer (≤ 10 μm), a surface structuring of this layer as well as a backside metal reflector, this technique achieves an increase of the light extraction efficiency.

The requirements placed on the luminescence converter material are high conversion efficiency, high stability against oxygen and moisture, low efficiency loss at higher temperatures (thermal quenching) and a suitable, as well as easily adjustable emission wavelength. As a rule, inorganic phosphors are used for the conversion. The converter materials consist of a few micrometers of crystallites, which are poured into epoxy resin or a silicone matrix. The crystallites contain optically active centers, usually the rare earths Cer (Ce ³⁺ ) or Europium (Eu ²⁺ ). The most frequently used converter in the yellow spectral range is YAG: Ce (cerium-doped yttrium-aluminum garnet, Y ₃ Al ₅ O ₁₂ : Ce ³⁺ ). For color matching, as well as for producing warm white, low color temperatures, phosphor mixtures are also used which also convert the LED pump light into the red and cyan spectral ranges. Novel nitridic phosphors, such. B. red emitting nitrosilicates (M ₂ Si ₅ N ₈ , M: alkaline earth metal (Ba, Sr Ca)) and cyan-emitting oxynitrides (MSi ₂ O ₂ N ₂ ) show good long-term stability and are used in modern warm-white LUCO LEDs ,

In addition to the LUCO LED principle, it is also possible to produce white light with several differently colored LEDs in one module. The light is composed of at least three LEDs in the colors red, green and blue. However, the performance of such RGB modules is currently limited by the green LEDs. These are far less efficient than red or blue LEDs and lead to the so-called "green gap" in the wavelength range between 520 and 600 nm. An alternative approach for the highly efficient generation of light in this "green gap" is the full conversion of LED pump light by means of suitable converter materials. This full conversion can be done with phosphors but with limited emission wavelength tunability and a broad emission spectrum compared to LEDs.

In the US 2010/0283074 A1 describes a semiconductor converter material based on the II-VI compound semiconductor, which is produced as a thin layer and connected via a connecting layer directly to the surface of the LED chips. This is easily possible in this case, since the substrates on which the semiconductor layers of the II-VI compound semiconductors are grown can be very easily removed by a wet-chemical etching step after connection to the LED chips. A weak point of II-VI semiconductors, however, is the low material stability, which leads to rapid aging in optoelectronic components. The structures are also usually based on cadmium-zinc selenide (CdZnSe), with both CdSe and ZnSe being classified as toxic.

From Galler et al., "Green high-power light sources using InGaN multi-quantum-well structures for full conversion", Phys. Status Solidi C 8 (2011), pages 2369 to 2371, the use of GaInN multi-quantum film structures as a semiconductor converter material for luminescence conversion light-emitting diodes known. However, the substrates on which such Group III nitride semiconductor materials are grown, in particular sapphire substrates, can not be separated from the grown semiconductor layer with a simple etching step. The sapphire substrate with the GaInN multi-quantum well converter structure is therefore thinned in the preparation of the luminescence conversion LEDs only to about 100 microns. The resulting thin plate with the grown converter layer is then adhered to the pump LED chips. Due to the smooth interfaces is the Light extraction, however, limited by total reflection. Also, the heat dissipation is not optimal due to the poor thermal conductivity of the remaining sapphire substrate.

The DE 103 54 936 A1 describes a radiation-emitting semiconductor component which has a semiconductor body with a semiconductor conversion layer arranged thereon. The semiconductor conversion layer is firstly produced on a substrate and applied with the substrate to the semiconductor body or a carrier layer located thereon. Subsequently, the substrate is removed by mechanical stress, selective etching or a laser stripping process.

The object of the present invention is to specify a method for producing a luminescence conversion light-emitting diode which fulfills the above-mentioned requirements for luminescence converter materials and, in particular, achieves a high light output of the luminescence conversion light-emitting diode.

Presentation of the invention

The object is achieved by the method according to claims 1 and 2. Advantageous embodiments of the proposed method for producing a luminescence conversion light-emitting diode are the subject of the dependent claims or can be found in the following description and the exemplary embodiments.

The luminescence conversion light-emitting diode that can be produced by the method comprises a light-emitting diode chip with a layer of a semiconductor converter material based on group III nitrides, wherein the layer without a substrate is applied to the light-emitting diode chip. The use of a semiconductor converter material based on Group III nitrides fulfills the requirements for high conversion efficiency, high stability against oxygen and moisture, low efficiency loss at higher temperatures and an adjustable emission wavelength. Such a converter material, for example based on GaInN with corresponding multi-quantum well structures, is, for example, from the above publication by Galler et al. known. The emission wavelength is adjusted by the composition and dimensioning of the quantum well structures. By arranging a thin layer of this semiconductor converter material on the surface of the pump LED chip without the substrate on which the layer is grown, a high luminous efficacy of such a luminescence conversion LED is achieved, as described below.

In a particularly advantageous embodiment, the converter layer is roughened on the surface facing away from the LED chip. Through this structuring or roughening of the coupling-out surface of the converter layer or of the luminescence conversion light-emitting diode, total reflection is avoided and the light extraction efficiency is thereby significantly increased again.

By completely removing the substrate on which the semiconductor converter layer is grown, in this case usually a sapphire substrate, the efficiency of white and monochromatic LUCO LEDs based on group III-nitride luminescence converters can be significantly improved. Since the semiconductor layer is very thin, for example, only about 5 microns thick, it comes to multiple reflections in the converter layer, through which the coupling of the light is favored. Due to the complete removal of the substrate, improved cooling of the component is achieved, since in particular sapphire represents a poor conductor of heat. In addition, removal of the substrate reduces the overall thickness of the component, which is advantageous not only in light extraction but also in the package, and facilitates the attachment of primary and secondary lenses for beam forming. A further advantage of the removal of the substrate is that an opening for the bonding wire to the LED chip can be introduced into the converter layer by means of laser micromachining or with other techniques.

For the production of such a luminescence conversion LED, a special method is required since semiconductor conversion materials based on group III nitrides can not be separated from their substrate by simple wet-chemical etching processes after connection to the LED chip. The transfer of the very thin semiconductor converter layer without substrate to the pump LED is not possible because this thin, for example, only about 5 .mu.m thick layer is not mechanically stable enough to be handled as a free-standing structure. Above all, the group III nitrides also do not allow roughening of the decoupling surface after application to the LED chip, since the aggressive etching process required for roughening the layer surface for these materials would damage the underlying LED structure.

For the application and roughening of this thin semiconductor layer without the required substrate for growth on the pump LED chips, hereinafter also referred to different from other substrates used as the base substrate, therefore, the following two variants of the method are proposed. The two process variants are not based on semiconductor conversion layers based on group III nitrides but can also be used for other converter materials.

In the first proposed method for producing a luminescence conversion LED, first of all a layer of the converter material is produced, in particular grown, on a base substrate. On the layer of the converter material is then applied via a first connection layer, for example by means of a polymer adhesive, a shadow mask. The shadow mask has a distribution of holes (holes) larger in diameter than the provided LED chips. The shadow mask is mechanically stable to handle without additional support. Instead of the shadow mask, another support structured in accordance with openings can also be used. The base substrate is then removed with a laser lift-off process, as he, for example, from the DE 196 40 594 A1 is known. By removing the base substrate, a surface of the layer of the converter material is exposed. This surface is then roughened, for example with a suitable wet-chemical etching process. The layer of converter material is then exposed on the opposite side in the openings of the shadow mask, if it is covered there by the first connection layer. Subsequently, the LED chips are inserted into the openings of the shadow mask and connected via an optically transparent second bonding layer with the layer of the converter material. In this case, the optical transparency is to be understood in conjunction with the emission wavelength of the light-emitting diode for which the second connection layer must be transparent. The connection is of course such that the light-emitting surface of the LED chip is connected to the layer of the converter material. The individual luminescence conversion LEDs obtained thereby are then separated. This can be done, for example, by means of laser micromachining. But other techniques can be used for this purpose.

With this method, thin, non-self-supporting layers of the converter material can be applied to pump LED chips after detachment from the base substrate required for the production. The method thus makes it possible, above all, to produce the proposed luminescence conversion LEDs with a thin converter layer based on group III nitrides. The production of such a layer usually requires a sapphire substrate that can not be detached by a simple wet chemical etching process. Through the use of the shadow mask, however, a transfer of the thin semiconductor layer is made possible on the LED chips. In an advantageous embodiment of the proposed method, the mechanical stability is increased even further by applying an auxiliary substrate to the shadow mask via a third connection layer prior to removal of the base substrate and removing it again after removal of the base substrate. This technique further increases the stability of the thin converter layer including the shadow mask in the laser lift-off process.

A second alternative of the proposed method utilizes an auxiliary substrate for transferring the thin layer of the converter material to the provided LED chips. Again, the first layer of the converter material is produced on a base substrate, in particular grown. The layer is then subdivided on the base substrate by structuring into a plurality of separate fields or islands, which are adapted to a size of the LED chips. The size of these fields or islands thus corresponds to the size of the later applied to the LED chips converter layer. The structuring can be carried out by known methods such as dry etching or laser micromachining. A first auxiliary substrate is then applied to the layer of converter material structured in this way via a first connection layer. The base substrate required for the formation of the layer is then removed by means of a laser lift-off process. By removing the base substrate, a surface of the structured layer of the converter material is exposed. This surface is then first roughened, for example with a suitable wet-chemical etching process, and then connected via a second connection layer with a second auxiliary substrate. The first auxiliary substrate is then removed. Finally, as in the case of the first method alternative, the LED chips are connected to the fields of the structured layer of the converter material via an optically transparent third connection layer. Alternatively, the wafer from which the light-emitting diode chips are later separated can also be connected to the fields of the structured layer of the converter material. The distances of the fields must be adapted to the distances of the areas for the LED chips on the wafer. Subsequently, the second auxiliary substrate is removed so that the individual luminescence conversion LEDs are obtained. In the case of the wafer-level connection, the individual luminescence conversion LEDs are then separated.

The compounds with an auxiliary substrate or the shadow mask are carried out in the proposed method alternatives via suitable connecting layers, for example suitable polymer adhesive. These are chosen so that the auxiliary substrates then easily detach again by simple wet-chemical etching processes or other processes, for example by thermal action to let. A replacement of the shadow mask is not required, since the size of the openings of the shadow mask during subsequent separation usually no remnants of the shadow mask must be replaced.

The two alternative methods are particularly suitable for applying a thin layer of preferably ≦ 10 microns layer thickness of a semiconductor converter material based on group III nitrides on the light-emitting surface of LED chips. The LED chips can be produced in a known manner according to the prior art and emit preferably in the blue or ultraviolet wavelength range. An example of the production of such LED chips is from the above-mentioned publication by A. Laubsch et al. known. As group III nitrides InN, GaN and AlN and their mixed crystals or alloys are preferably used.

The LUCO LEDs with group III nitride-based semiconductor converters which can be produced by the proposed methods can be advantageously used for use as green and also white light emitters. By using thin-film converter structures and the associated possibility of surface structuring, for example, the luminous efficacy of LUCO LEDs with GaInN-based luminescence converters can be significantly increased. Such luminescence conversion LEDs can be used for many applications, in particular for general lighting and for high-quality lighting solutions.

Brief description of the drawings

The two proposed alternative methods for producing a luminescence conversion LED will be explained in more detail below with reference to exemplary embodiments in conjunction with the drawings. Hereby show:

1 a schematic representation of an exemplary process flow for producing the luminescence conversion LED according to the first alternative method; and

2 a schematic representation of an exemplary process flow for the production of the luminescence conversion LED according to the second alternative method

Ways to carry out the invention

The first alternative method for producing a luminescence conversion LED is exemplified by the in 1 again explained steps.

The semiconductor converter layer 1 is doing in a known manner on a sapphire substrate 2 produced by epitaxial growth as a GaN layer ( 1a ). The sapphire substrate 2 becomes with the grown converter layer 1 by means of a polymer adhesive 3 on a shadow mask 4 glued ( 1b ). The shadow mask 4 may, for example, have a thickness of about 100 microns in order to achieve sufficient mechanical stability. Such shadow masks can be handled analogously to a thin wafer.

Optionally, the arrangement is over another polymer adhesive layer 5 on a carrier substrate 6 glued to the mechanical stability for the subsequent separation of the sapphire substrate 2 increase again ( 1c ). Subsequently, the sapphire substrate 2 removed by laser lift-off, leaving a surface of the semiconductor converter layer 1 exposed ( 1d ).

In the present example, this exposed surface becomes the semiconductor converter layer 1 subsequently roughened wet-chemically by means of KOH etching in order to increase the light extraction efficiency of the later luminescence conversion LED. The roughened surface is in 1e indicated. The optional carrier substrate 6 is then cured by thermal or chemical release of the polymer adhesive layer 5 away ( 1f ). That in this example at the bottom of the semiconductor converter layer 1 in the openings of the shadow mask 4 still existing material of the polymer adhesive layer 3 is removed by the back mask openings with a corresponding solvent, so that the GaN-based converter layer 1 only on the webs of the shadow mask 4 is attached ( 1g ).

In the case of non-bond wire-free LED chips, an opening for the front-side bonding wire can be made into the semiconductor converter layer by means of laser micro-machining 1 be introduced as in the 1h is indicated. Then the provided LED chips 7 with an optically transparent connection material 8th , For example, silicone, on the converter layer 1 glued ( 1i ). With a laser micromachining system is now the converter layer 1 severed, as in 1j is indicated. The luminescence conversion LEDs 9 with the transferred GaN conversion layers can subsequently be detached from the shadow mask and are available as separate components ( 1k ). The attached to the LED chips bonding wire is also indicated in this figure.

2 shows an example of the procedure in the second variant of the proposed method. In this process variant, a temporary intermediate carrier for the transmission of the semiconductor conversion layer 1 on the LED chips 7 used. Again, first, the semiconductor converter layer 1 on the sapphire substrate 2 generated in a known manner ( 2a ). Next, the GaN converter layer 1 by dry etching or micromachining the LED chip size accordingly divided into fields as shown in FIG 2 B is indicated. This is followed by a wafer bonding step with a KOH-resistant polymer 10 ( 2c ). In this step, a carrier substrate 11 with the structured semiconductor converter layer 1 connected. After that, the sapphire substrate becomes 2 removed by laser lift-off ( 2d ).

As already related to 1 1, the surface of the patterned semiconductor converter layer exposed thereby becomes 1 roughened by KOH etching to increase the subsequent light extraction efficiency wet-chemically 2e ). Subsequently, at wafer level becomes a temporary carrier 12 over a polymer adhesive layer 13 on the roughened surface of the semiconductor converter layer 1 glued ( 2f ) and the carrier substrate 11 selectively removed ( 2g ). The LED chips 7 be with optically transparent material 8th on the GaN converter layer 1 glued on, like this 2h is apparent. Subsequently, the temporary carrier 12 removed, leaving the luminescence conversion LEDs 9 are available as separate components ( 2i ).

This in conjunction with 2 described method variant can also be applied to wafer level instead of chip level. Here, instead of the LED chips 7 used a thin-film LED wafer. The individual luminescence conversion LEDs must then subsequently be separated by a suitable separation step.

LIST OF REFERENCE NUMBERS

1

Semiconductor converter layer

2

Sapphire substrate

3

Polymer adhesive layer

4

shadow mask

5

Polymer adhesive layer

6

carrier substrate

7

LED chip

8th

optically transparent connecting material

9

Luminescence conversion LED

10

polymer

11

carrier substrate

12

temporary carrier

13

Polymer adhesive layer

Claims (7)

Method for producing a luminescence conversion light-emitting diode, in which a layer ( 1 ) from a converter material on light-emitting diode chips ( 7 ) is applied by - the layer ( 1 ) from the converter material first on a base substrate ( 2 ), - on the layer ( 1 ) from the converter material via a first connection layer ( 3 ) a self-supporting shadow mask ( 4 ) or another correspondingly structured carrier having a distribution of openings which is larger in diameter than the light-emitting diode chips (US Pat. 7 ), - the basic substrate ( 2 ) is removed by means of a laser lift-off process, - the layer ( 1 ) from the converter material after removal of the base substrate ( 2 ) is roughened on the thus exposed surface, - the layer ( 1 ) from the converter material in the openings of the shadow mask ( 4 ) or the correspondingly structured carrier if it is exposed there by the first connecting layer ( 3 ), - the light-emitting diode chips ( 7 ) in the openings of the shadow mask ( 4 ) or the correspondingly structured carrier and via an optically transparent second connecting layer ( 8th ) with the layer ( 1 ) are connected from the converter material, and - the individual luminescence conversion light-emitting diodes ( 9 ) are then separated. Method for producing a luminescence conversion light-emitting diode, in which a layer ( 1 ) from a converter material on light-emitting diode chips ( 7 ) is applied by - the layer ( 1 ) from the converter material first on a base substrate ( 2 ), - the layer ( 1 ) is divided by structuring into a plurality of separate fields, which corresponds to the size of the light-emitting diode chips ( 1 ), - on the structured layer ( 1 ) from the converter material via a first connection layer ( 10 ) a first auxiliary substrate ( 11 ), - the base substrate ( 2 ) is removed by means of a laser lift-off process, - the layer ( 1 ) from the converter material after removing the base substrate ( 2 roughened on the thus exposed surface and - via a second connecting layer ( 13 ) with a second auxiliary substrate ( 12 ), - the first auxiliary substrate ( 11 ), - the light-emitting diode chips ( 7 ) or a wafer from which the light-emitting diode chips ( 7 ) are separated later, via an optically transparent third compound layer ( 8th ) with the fields of the structured layer ( 1 ) are connected from the converter material, and - the second auxiliary substrate ( 12 ) Will get removed. Method according to claim 1, characterized in that before the removal of the basic substrate ( 2 ) an auxiliary substrate ( 6 ) via a third connection layer ( 5 ) on the shadow mask ( 4 ) or the correspondingly structured carrier and after removing the base substrate ( 2 ) is removed again. Method according to one of claims 1 to 3, characterized in that the converter material used is a semiconductor converter material and the layer ( 1 ) of the semiconductor converter material on a sapphire substrate as a base substrate ( 2 ) is produced. Method according to one of claims 1 to 4, characterized in that a semiconductor converter material based on group III nitrides as converter material on the base substrate ( 2 ) is produced. Method according to one of claims 1 to 5, characterized in that the roughening of the layer ( 1 ) is carried out from the converter material on the exposed surface by KOH etching. Method according to one of claims 1 to 6, characterized in that the layer ( 1 ) from the converter material with a thickness of ≤ 10 μm on the base substrate ( 2 ) is produced.

Images