WO2007031929A1

WO2007031929A1 - Method for manufacturing led wafer with light extracting layer

Info

Publication number: WO2007031929A1
Application number: PCT/IB2006/053204
Authority: WO
Inventors: Hans Van Sprang; Marcus A. Verschuuren; Martinus P. J. Peeters
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2005-09-16
Filing date: 2006-09-11
Publication date: 2007-03-22
Also published as: TW200717872A

Abstract

A method for providing an inorganic LED wafer with an inorganic structured light extraction layer, comprising providing a coating of a sol-gel precursor on said wafer, ensuring that the surface of said sol-gel coating is susceptible to embossing, embossing a structure in the sol-gel coating using soft-lithography, and curing the sol-gel layer, thereby forming said light extraction layer. The invention is based on the understanding that sol-gel precursors, because of their properties before a final curing step, constitute a suitable base material for soft lithography, and that their optical properties make them suitable for use as optical components. Compared to traditional optical lithography the soft lithography requires fewer processing steps and is thus faster and less expensive, both in terms of production cost as well as investment cost.

Description

LED wafer with light extracting layer and method for its manufacture

The present invention relates to providing LEDs with an embossed or imprinted inorganic layer for enhanced extraction of light. More specifically, this layer is formed by soft-lithography of a sol-gel precursor.

The use of light-emitting diodes (LED's) is ever increasing. LED's have during the last decade or so proven to be a reliable source of radiation in providing a robust, reproducible construction and an efficient transformation of energy.

With the increasing use of LED's as a radiation source follows the need for methods for collection/extraction of the radiation emitted by the diodes. In this context some different approaches have been used. The first includes the use of conventional optical components, such as PMMA- lenses. An important drawback with this approach is the significant differences in refractive index between the materials in the lens and the high index materials in the substrate and active layers. These and other problems have led to a second approach, in which optical structures, e.g. in the form of distributed Bragg reflectors (DBR), are applied to the LED in order to increase light extraction.

There are several drawbacks regarding the latter prior-art approach. One is that the methods, though feasible, are not well adapted to large-scale production, both in terms of physical scale and production scale. In literature there are a number of cases where optical structures have been applied to enhance light extraction. Most of the time these structures consist of DBR type of reflectors, which actually are optical multilayers using a set of materials with alternating high and low index. It has been shown that a periodically structured silver mirror can increase light extraction. In this case the patterning was, however, done by using an electronic beam pattern generator (EBPG) and a resist directly on the LED. This is of course inconceivable for application to actual production.

Thus, there is a need for a commercially applicable method of applying an optical structure with small dimensions, including sub-wavelength dimensions, to a LED. It is an object of the present invention to eliminate the problems mentioned above and to provide a commercially feasible way to providing a LED with an optical structure having small, typically sub-wavelength, dimensions.

This and other objects are achieved with a method for providing an inorganic LED wafer with an optical structure adapted to enhance light extraction from said LED, comprising providing a coating of a sol-gel precursor on said wafer, ensuring that the surface of said sol-gel coating is susceptible to embossing, embossing a structure in the sol-gel coating using soft-lithography, and curing the sol-gel layer, thereby forming said light extraction layer.

According to the invention, an inorganic light extraction layer can be applied by soft-lithography directly to the whole wafer from which the LEDs will be produced. The wafer can be sapphire or any other suitable material. The fact that the layer is applied on a macroscopic scale on the entire LED wafer makes it industrially favorable (less expensive) than to provide the structures on each LED separately.

By using soft lithographic techniques, such as wave printing or vacuum embossing, it is possible to imprint structures in these layers with various aspect ratios and details as small as 30 nm.

The invention is based on the understanding that sol-gel precursors, because of their properties before a final curing step, constitute a suitable base material for soft lithography, and that their optical properties make them suitable for use as optical components. Compared to traditional optical lithography the soft lithography requires fewer processing steps and is thus faster and less expensive, both in terms of production cost as well as investment cost.

Document US 2004/0144976 discloses use of a sol-gel layer in an OLED. Also in this case the properties of the sol-gel are used to provide a structured layer with suitable optical properties. However, the sol-gel layer in US 2004/0144976 is provided on the substrate before the active layers of the OLED are deposited, and is used to create identical structures in each deposited layer. It is the structuring of the layers in the active stack that achieves the desired effect. According to the present invention, the sol-gel layer is instead used to provide a light extraction layer formed outside the active stack of an inorganic LED. The term "sol-gel precursor" generally relates to a metal-alkoxide compound, also in combination with colloidal particles, in particular colloidal silica particles (e.g. Ludox). A preferred metal-alkoxide is an organosilane compound, forming a hybrid sol-gel precursor. A hybrid sol-gel precursor comprising an organosilane compound is understood to be a compound comprising silicon, which is bond to at least one non-hydro lysable organic group, and 2 or 3 hydroly sable organic groups.

In particular, the hybrid sol-gel precursor may comprise an organosilane compound from the group of alkyl-alkoxysilanes. The silane can be mono-organically modified using for example methyl, ethyl or phenyl as organic modifier. Mono-organically modified is to be construed, as one of the four covalent bonds of the silicon is a Si-C bond. In this case, the remaining three bonds are Si-O bonds. Examples of preferred sol-gel precursors comprise methyl-tri-methoxy-silane (MTMS), which is a mono-methyl-modified silane, and methyl-triethoxysilane (MTES). After suitable processing, MTMS results in a bonding material comprising a matrix having the basic structure CH₃-Si-O_{1 5} (i.e. a silsesquioxane). The matrix has a relatively high elasticity due to the fact that the silicon atoms are only threefold cross-linked to each other. Hybrid layers produced using sol-gel precursors such as MTMS and MTES are known to have excellent temperature stability up to at least 400 degrees C in air. Other suitable precursor materials include T-resins, such as Silres 610 or Silres 603 from Wacker Chemie GmbH.

The precursor can further comprises an oxide including at least one element selected from the group consisting of Si, Al, Ga, Ti, Ta, Ge, P, B, Zr, Y, Sn , Pb, and Hf. The oxide serves to increase the light extraction layer's index of refraction, which in turn enhances the light coupling capability of the layer. Such particles can be used to introduce optical scattering in the layers, or allow thicker layers to be deposited. These particles can also be of fluorescent or phosphorescent nature, which allows for very special effects in terms of enhanced emission or angular distribution of the emission.

It is noted that the use of 2- or 3 -dimensional photonic crystals as optical structures for enhancing light extraction has been described theoretically but has previously been almost impossible to realize on a practical scale. The reason is the very small dimensions required in lateral direction combined with the aspect ratio dimensions in the vertical direction. It is especially the sub- wavelength nature of the lateral dimensions that precludes use of optical lithography and has necessitated EBPG based patterning. With the use of sol-gel based layers and simple structuring techniques according to the present invention it is feasible to make these structures on large areas and highly reproducible. The embossing can be performed using wave printing, which makes it feasible to use soft lithography on a larger scale, in terms of area and thus production volume, by ensuring a good contact between a stamp and the sol-gel coating, as well as good release between the two. Alternatively, the embossing can be performed using vacuum embossing. The sol-gel precursor preferably contains a high boiling solvent, and the embossing can then be performed before the solvent has evaporated completely, so that the sol-gel precursor is still soft and deformable. As sol-gel layers as disclosed above only require curing at very limited temperatures (200-300 degrees Celsius), the sol-gel coating can be applied directly on top of an active stack (light emitting layer) on the LED wafer. The active stack may or may not be deposited on a substrate.

Alternatively, the sol-gel coating is applied directly onto a substrate wafer. The emitting layer can then be deposited on the other side of this wafer, before or after the sol-gel process, or be deposited on top of the embossed and cured light extraction layer.

The structure can be formed as a lens, e.g. a Fresnel lens. When decreasing the size, arrays of such structures can be made, and when the overall dimensions become of the order of the wavelength of the light special structures become available like moth-eye extraction layers or photo randomization layers.

The structure can also be formed as a photonic band-gap (PBG) structure. Such a structure consists of sub- wavelength patterned layers where the structure precludes light transmission in certain directions while enhancing it for other wavelengths or directions.

According to one embodiment, the sol-gel structure is filled with another material, and the sol-gel layer is then removed. The material can be a LED related material (e.g. GaN) or any other material that can be applied in physical or chemical way. The sol-gel structure is thus used as a kind of mould, for forming a structure of another material. This allows forming a patterned structure that can consist of a wide range of materials, without the need for traditional lithographic treatment. In other words, materials that normally must be treated with traditional techniques, such as optical lithography, can instead be structured using indirect soft lithography, as long as they can be applied to the sol-gel layer in a physical or chemical way. When the structured sol-gel material is used as a mould only, organic materials can sometimes be used instead of the sol-gel materials, provided that the temperatures involved are not too high. This and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing a currently preferred embodiment of the invention.

Figures la-c are schematic side views of a LED provided with a light extraction layer according to different embodiments of the invention.

Figure 2 shows an embodiment of the method according to the invention. Figure 3a shows the principle of wave printing. Figure 3b shows the principle of vacuum embossing. Figures 4a-c are SEM images showing three examples of structures suitable for realization according to the invention.

Figures la-c shows examples of how the light extraction layer according to the invention can be arranged on a LED 2. In fig Ia, the sol-gel layer 1 is provided on the opposite side of the substrate 3 on which the active layer 2 of the LED are deposited. In figures Ib and Ic, the sol-gel layer 1 is applied directly on the active layer 2. In fig Ib, the active layer 2 has been deposited on a substrate 3, in fig Ic the LED is a substrate free LED.

Figure 2 shows an embodiment of the method according to the invention. First, in step Sl, a thin coating 1 of sol-gel is applied onto the desired area. The sol-gel can be applied with a suitable method, such as spin coating, spray coating or screen printing. As indicated in fig la-c, the area can be the active layer of a LED, or a substrate on which the active layer has been deposited, or on a substrate on which the active layer will be deposited.

The sol-gel layer 1 is then pre-dried in step S2, during which a gelation process sets in to create a network or matrix structure. In order to allow embossing of the surface of the sol-gel layer 1, it is ensured that this surface remains soft. In the present example, the sol-gel has been provided with a small amount (5-15%) of a high boiling solvent. During the pre-drying, a controlled amount of high boiling solvent is left in the coating, and keeps the coating soft and deformable.

Then, in step S3, a structure is embossed in the sol-gel layer 1 using a soft lithography technique. Different ways to perform such lithography will be described below. Finally, the sol-gel is cured in step S4, to leave a structured silicon layer, adapted to act as a light extraction layer of the LED.

The embossing can be performed using a wave printer 10 as schematically illustrated in Fig. 3 a. The wave printer uses a patterned PDMS (poly-di-methyl-siloxane) rubber stamp 11 that, by means of air pressure provided through grooves 12, is forced in and of contact with the sol-gel layer 1. The contact is executed with a wavelike motion, as illustrated in Fig. 3a, in order to reduce the amount of ambient gas trapped between the stamp 11 and the sol-gel layer 1 during the contacting step, as well as to reduce the forces involved when the stamp 11 is detached from the sol-gel layer. The pressure underneath the stamp 11 is kept for some time to ensure that the pattern of the stamp 11 is replicated into the sol-gel layer 1. The wave printing principle is described in WO03/099463.

Another way to perform the soft lithography is by vacuum embossing, shown in fig 3b. Here, the sol-gel is allowed to pre-dry and is then placed in a vacuum chamber together with the stamp 15. Pressure is decreased, and the stamp is released and allowed to abut the wet sol-gel layer 1. When pressure is again raised, sol-gel is pressed into the cavities of the stamp, and remaining air/solvent diffuses into the rubber. The vacuum embossing technique requires a sol gel- lacquer, which is free from water and has a large percentage of organic solvents. In the following, a few examples of structures suitable for imprinting on the sol-gel applied on the LED will be described, with reference to the SEM-images in figures 4a-c.

Fig. 4a shows micro lenses in a silica sol-gel. Such micro lenses can be used for enhanced extraction/collection of light from the inorganic LED. Fig. 4b shows a two-dimensional structure for extraction of light (moth-eye structure), comprising nano bumps that reduce reflectivity and enhance out-coupling of light. Fig. 4c shows a 2-dimensional PBG structure comprising a hexagonal grid of sol-gel needles. In this structure the two-dimensional PBG is similar to the construction shown in the sol-gel hexagonal pattern of needles and so this structure does not need extra lithography steps and etching procedures but can be applied in a sol-gel layer (which can also be TiO2 if higher index material is required) at the wafer level.

It should be noted that the above structures could be embossed in the sol-gel over the whole area of a led substrate wafer according to the above described method. As mentioned, the structuring can be done when the active layers have already been deposited on the substrate, but can also been done before deposition of the active layers.

Claims

CLAIMS:

1. A method for providing an inorganic LED wafer with an inorganic structured light extraction layer, comprising: providing a coating of a sol-gel precursor on said wafer (step Sl), ensuring that the surface of said sol-gel coating is susceptible to embossing (step S2), embossing a structure in the sol-gel coating using soft-lithography (step S3), and curing the sol-gel layer (step S4), thereby forming said light extraction layer.

2. A method according to claim 1, wherein said sol-gel precursor comprises an organosilane compound and silica particles.

3. A method according to claim 2, wherein said organosilane compound belongs to the group of alkyl-alkoxysilanes.

4. A method according to one of the preceding claims, wherein the sol-gel precursor comprises an oxide including at least one element selected from the group consisting of Si, Al, Ga, Ti, Ta, Ge, P, B, Zr, Y, Sn , Pb, and Hf.

5. A method according to one of the preceding claims, wherein said embossing is performed using wave printing.

6. A method according to one of claim 1-4, wherein said embossing is performed using vacuum embossing.

7. A method according to any one of the preceding claims, wherein the sol-gel precursor contains a high boiling solvent, and wherein said embossing is performed before said solvent has evaporated.

8. A method according to any one the preceding claims, wherein said sol-gel layer is provided directly on an inorganic light emitting layer.

9. A method according to any one claims 1-7, wherein said sol-gel coating is provided on one side of a substrate wafer, and an emitting layer is deposited on the other side of said substrate wafer.

10. A method according to any one claims 1-7, wherein said sol-gel coating is provided on a substrate wafer, and an emitting layer is deposited on said embossed and cured layer.

11. A method according to any one of the preceding claims, wherein said structure is formed as a lens.

12. A method according to any one of the preceding claims, wherein said structure is formed as a photonic band-gap (PBG) structure.

13. A method according to any one of the preceding claims, further comprising filling the structure in the sol-gel layer with another material, and removing said sol-gel layer.

14. An LED wafer comprising an inorganic light emitting layer, characterized in that it is provided with a structured light extraction layer formed by soft-lithography of a sol- gel coating.

15. An LED wafer according to claim 14, wherein said light extraction layer comprises one of the group consisting of lenses, micro-lenses, Fresnel lenses, moth-eye extraction layers, photo-randomization layers or photonic band gap structures.