The present invention relates to a stencil, often referred to as a printing screen or foil, for printing patterns of a printing medium onto substrates, in particular wafers or transfer carriers.

The present invention has particular application in the printing of conversion phosphors onto wafer dies, such as in depositing yellow down-conversion phosphors, for example, YAG-Ce, for the down conversion of UV and/or blue light from light-emitting devices, such as LEDs or lasers, to provide white light.

In printing such phosphors, it is important that the material be deposited with a high degree of uniformity, in order to achieve uniform luminescence and thus color temperature.

Conventionally, down-conversion phosphors are dispensed using dispensing devices, and attempts to print phosphors using stencils have suffered from the problem of exhibiting low wafer yields, typically around 50%, in that the prints on a significant number of prints do not have the required uniformity, resulting in significant wastage from each printed wafer.

It is an aim of the present invention to provide an improved stencil for printing patterns of printing medium onto substrates, in particular wafers or transfer carriers, and especially in the printing of down-conversion phosphors onto wafers, such as sapphire or silicon wafers, in the fabrication of light-emitting devices for emitting white light.

In one aspect the present invention provides a stencil for printing a pattern of deposits on a substrate, wherein the stencil comprises an e!ectroformed metal sheet which has a first layer which includes an apertured region through which a printing medium is applied in a printing operation, and a second layer which overlies a substrate to be printed and includes a plurality of apertures, wherein the apertures in the second layer extend across and beyond the apertured region in the first layer, whereby the second layer includes a plurality of through apertures in registration with the apertured region of the first layer, each having a pattern corresponding to that to be printed on the substrate, and a plurality of blind apertures disposed adjacent and outwardly of the apertured region in the first layer.

In one embodiment the metal sheet is formed of nickel or a nickel alloy.

In one embodiment the layers of the stencil are integrally formed.

In one embodiment the layers are formed of the same material.

In another embodiment the layers are formed of different materials.

In one embodiment the apertured region corresponds in shape and size to the substrate to be printed.

In one embodiment the apertured region is circular in shape.

In one embodiment the apertured region has the form of a grid which comprises orthogonally-arranged web elements, which together define apertures therebetween.

In one embodiment the apertures of the first layer are rectangular.

In one embodiment the web elements of the first layer have a width of from about 10 pm to about 120 pm, preferably from about 20 pm to about 110 pm, more preferably from about 30 pm to about 100 pm, and more preferably about 30 pm or about 100 pm. In one embodiment the web elements of the first layer have a width of from about 10 μηη to about 40 pm, preferably from about 20 pm to about 40 pm, and more preferably about 30 pm.

In one embodiment the web elements of the first layer have a width of from about 80 pm to about 120 pm, preferably from about 90 pm to about 110 pm, and more preferably about 100 pm.

In one embodiment the apertures of the first layer have an area of at least about 0.001 mm ² , preferably from about 0.001 mm ² to about 1 mm ² , more preferably at least about 0.0015 mm ² , still more preferably from about 0.0015 mm ² to about 1 mm ² , yet more preferably at least about 0.0025 mm ² , yet still more preferably from about 0.0025 mm ² to about 1 mm ² , and stiil yet more preferably not more than about 0.25 mm ² .

In one embodiment the apertures of the first layer have side iengths of at least about 50 pm, preferably at least about 100 pm, more preferably at least about 250 pm, and still more preferably not more than about 1 mm.

In one embodiment the first layer has a thickness of from about 10 pm to about 120 pm, preferably from about 20 pm to about 110 pm, more preferably from about 30 pm to about 100 pm, and still more preferably about 30 pm or about 100 pm.

In one embodiment the first layer has a thickness of from about 20 pm to about 60 pm, preferably from about 20 pm to about 50 pm, more preferably from about 25 pm to about 35 pm, and still more preferably about 30 pm.

In one embodiment the first layer has a thickness of from about 80 pm to about 120 pm, preferably from about 90 pm to about 110 pm, and preferably about 100 pm. In one embodiment the apertures in the second layer have a substantially square form, separated by orthogonally-arranged web elements.

In one embodiment the web elements of the second layer have a width of from about 100 pm to about 200 pm, preferably from about 100 pm to about 150 μηι.

In one embodiment the apertures in the second layer are arranged in the form of a regular array.

In one embodiment the apertures in the second layer repeat laterally outwardly beyond the apertured region of the first layer.

In one embodiment the apertures of the second layer extend laterally beyond the apertured region of the first layer by a distance of at least about 2 mm, preferably from about 2 mm to about 30 mm, more preferably from about 2 mm to about 20 mm, still more preferably at least about 5 mm, yet more preferably from about 5 mm to about 20 mm, and still more preferably from about 5 mm to about 10 mm.

In one embodiment the substrate is a wafer, preferably a silicon or sapphire wafer.

In another embodiment the substrate is a transfer carrier for transferring the prints to a wafer, preferably a silicon or sapphire wafer.

In another aspect the present invention provides a method of printing substrates with a pattern of deposits using the above-described stencil.

In one embodiment the method is for printing deposits of a down- conversion phosphor on a substrate, preferably a yellow down-conversion phosphor. In one embodiment the method comprises the steps of: providing a substrate; providing the above-described stencil over the substrate; applying print medium over the stencil, such that the print medium is forced through the apertures in the second layer and a pattern of deposits is printed on the substrate corresponding to the pattern of through apertures in the second layer of the stencil.

In one embodiment the substrate is a wafer, preferably a silicon or sapphire wafer, and the deposits are printed directly onto dies formed in the wafer without any intermediate transfer steps.

In a further aspect the present invention provides a method of fabricating a light-emitting device, comprising the steps of: performing the above- described printing step; and separating the printed dies of the wafer.

In one embodiment at least 90% of the printed dies of the wafer are selected, and further comprising the step of: providing each of the selected dies in device packaging to provide light-emitting devices.

In one embodiment the deposits on the selected dies of the wafer are not subjected to any surface thickness processing.

A preferred embodiment of the present invention will now be described hereinbelow by way of example only with reference to the accompanying drawings, in which :

Figure 1 illustrates a plan view of a stencil in accordance with a preferred embodiment of the present invention, mounted in a supporting frame;

Figure 2 illustrates an underneath view of the stencil of Figure 1 ;

Figure 3 illustrates a fragmentary, sectional perspective view of the stencil of Figure 1 (illustrated in an inverted orient from the operative orient); and Figure 4 illustrates a vertical sectional view (along section I-I in Figure 1) of the stencil of Figure 1.

Figures 1 to 4 illustrate a stencil 3 in accordance with a preferred embodiment of the present invention, mounted in a supporting frame 4, in this embodiment a VectorGuard ® frame (as supplied by DEK).

The stencil 3 comprises an electroformed metal sheet, in this embodiment of solid metal, here of nickel or a nickel alloy. In alternative embodiments the stencil 3 could be formed of other electroformable metals or alloys or combinations thereof.

As illustrated in Figures 3 and 4, the stencil 3 comprises a first, upper layer 5 over which a printing medium is applied in a printing operation, typically using a squeegee or an enclosed print head, and a second, lower layer 7, which overlies a substrate which is to be printed.

In this embodiment the layers 5, 7 of the stencil 3 are integrally formed. In one embodiment the layers 5, 7 are formed of the same material. In another embodiment the layers 5, 7 are formed of different materials.

The upper layer 5 includes an apertured region 11, in this embodiment of circular shape, which corresponds in shape and size to the substrate to be printed, and through which printing medium is delivered in a printing operation. It will be understood that the apertured region 11 could have any shape, for example, rectangular.

In this embodiment the apertured region 11 has the form of a grid, which comprises orthogonally-arranged web elements 15, 17, which together define apertures 19 therebetween, through which printing medium can be delivered. In this embodiment the apertures 19 are rectangular, typically square or oblong, but in other embodiments could have different shape, such as circular.

In this embodiment the web elements 15, 17 have a width of from about 10 Mm to about 120 μιτι, preferably from about 20 pm to about 110 pm, more preferably from about 30 m to about 100 pm, and more preferably about 30 pm or about 100 pm.

In one embodiment the web elements 15, 17 have a width of from about 10 pm to about 40 pm, preferably from about 20 pm to about 40 pm, and more preferably about 30 pm.

In another embodiment the web elements 15, 17 could have a width of from about 80 pm to about 120 pm, preferably from about 90 pm to about 110 pm, and more preferably about 100 pm

In this embodiment the apertures 19 have an area of at least about 0.001 mm ² , preferably from about 0.001 mm ² to about 1 mm ² , more preferably at least about 0.0015 mm ² , still more preferably from about 0.0015 mm ² to about 1 mm ² , yet more preferably at least about 0.0025 mm ² , yet still more preferably from about 0.0025 mm ² to about 1 mm ² , and still yet more preferably not more than about 0.25 mm ² .

In one embodiment the apertures 19 have side lengths of at least about 50 pm, preferably at least about 100 pm, more preferably at least about 250 pm, and still more preferably not more than about 1 mm.

In this embodiment the upper layer 5 has a thickness of from about 10 pm to about 120 pm, preferably from about 20 pm to about 110 pm, more preferably from about 30 pm to about 100 pm, and still more preferably about 30 pm or about 100 pm. In one embodiment the upper layer 5 has a thickness of from about 20 pm to about 60 μητι, preferably from about 20 pm to about 50 pm, more preferably from about 25 pm to about 35 pm, and still more preferably about 30 pm.

In another embodiment the upper layer 5 has a thickness of from about 80 pm to about 120 pm, preferably from about 90 pm to about 110 pm, and preferably about 100 pm.

The lower layer 7 includes a plurality of apertures 31, which each have a pattern corresponding to that to be printed on the substrate.

In this embodiment the apertures 31 each have a substantially square form, separated by orthogonally-arranged web elements 33, 35, but it should be understood that the apertures 31 could have any desired form.

In this embodiment the web elements 33, 35 have a width of from about 100 pm to about 200 pm, preferably from about 100 pm to about 150 pm.

In this embodiment the apertures 31 are arranged in the form of a regular array, with the apertures 31 being registered to dies on a substrate, in this embodiment a wafer.

The apertures 31 repeat laterally beyond the apertured region 11 of the upper layer 5 in a non-apertured region 37.

In this embodiment the apertures 31 extend laterally beyond the apertured region 11 by a distance of at least about 2 mm, preferably from about 2 mm to about 30 mm, more preferably from about 2 mm to about 20 mm, still more preferably at least about 5 mm, yet more preferably from about 5 mm to about 20 mm, and still more preferably from about 5 mm to about 10 mm. With this arrangement the apertures 31 in the non-apertured region 37 define blind apertures or recesses 31' in the lower surface of the stencil 3.

The present inventors have identified that, by extending the apertures 31 in the lower layer 7 beyond the apertured region 11 in the upper layer 5 to provide the blind apertures or recesses 31', the stencil 3 provides for significantly improved performance in printing across the entire substrate, and thus significantly-improved yield.

It has been found that, with this configuration, and in one example in the printing of a yellow down-conversion phosphor, the yield is remarkably increased to at least 90%, as compared to yields of about 70% for a stencil of the same design but having no blind apertures recesses 31', and, for some wafers, yields of 99% have been achieved.

Such is the improvement that it is not necessary to finish the surface of the prints, such as by lapping, to achieve a required thickness and thickness uniformity, or to check the thickness, where printed onto a transfer carrier, prior to transfer onto the dies of a wafer, as are done currently.

Finally, it will be understood that the present invention has been described in its preferred embodiment and can be modified in many different ways without departing from the scope of the invention as defined by the appended claims.